General Information. Individual Study Projects. Muon Lifetime Experiment. Today s Agenda. Wednesday: We have a few more slots to fill

Transcription

1 General Information Individual Study Projects We have a few more slots to fill Muon Lifetime Experiment We can use the experiment until April 18 Monday, April 8 we will go over the setup and take a look at the equipment in Smith Lab We have time for three groups: Group 1 Setup Tuesday, April 9; Finish Friday morning (4/12) Group 2 Setup Friday, April 12; Finish Monday morning (4/18) Group 3 Setup Monday April 15; Finish on April 18 Today s Agenda Postponed: Interaction of Particles with Matter Characteristics of a particle detector Scintillators Wednesday: Signals and Electronics

2 Functional Components of a Detector Decay scheme of 137 Cs

3 Functional Components of a Detector Characteristics Resolution Efficiency Sensitivity Deadtime

4 Resolution = E/E Energy Resolution The width arises because of fluctuations in the number of ionizations or excitations produced. If w is the energy needed to produce an ionization or excitation, one would expect, on average N = E/w Poisson process (ie variance = mean) = N If we take the resolution as the full width half maximum (FWHM) of the distribution we get R = 2.35 N /N = 2.35 (w/e) (the factor 2.35 relates the standard deviation of a Gaussian to its FWHM) Function of energy deposited; improves with higher energy Better resolution for smaller w (e.g. silicon detectors) Fano Factor f Poisson statistics can t be applied if all energy is absorbed. Fano found that the variance in this case is = F N F<1 for gases, semi-conductors -> greatly improves resolution

5 Absolute Efficiency Efficiency and Deadtime Function of geometry and the probability of interaction in the detector: tot = (events detected) / (events emitted by source) tot = int geometry Intrinsic Efficiency Fraction of events actually hitting the detector that are registered int = (events detected) / (events impinging on detector) Deadtime Some detectors require some time to process an event and might not be sensitive for new events during this time. If the count rate is sufficiently low this effect can be corrected. R true = true rate R measured = measured rate = detector deadtime

6 Electromagnetic Interaction of Particles with Matter Z 2 electrons, q= e 0 M, q=z 1 e 0 Interaction with the atomic electrons. The incoming particle looses energy and the atoms are excited or ionized. Interaction with the atomic nucleus. The particle is deflected (scattered) resulting in multiple scattering of the particle in the material. During these scattering events a Bremsstrahlung photons can be emitted. W. Riegler, Particle Detectors In case the particle s velocity is larger than the velocity of light in the medium, the resulting EM shockwave manifests itself as Cherenkov Radiation. When the particle crosses the boundary between two media, there is a probability of the order of 1% to produce an X ray photon, called Transition radiation.

7 Basic EM Interactions e + / e - Ionization de/dx ~ 1/ 2, z 2 de/dx E Bremsstrahlung de/dx ~ 1/m 2, z 4 de/dx E Photoelectric effect E Compton effect E Pair production E

8 Interaction of Particles with Matter Any device that is to detect a particle must interact with it in some way almost In many experiments neutrinos are measured by missing transverse momentum. E.g. e + e - collider. p total = 0, If Σ p i of all collision products is 0 neutrino escaped. Claus Grupen, Particle Detectors, Cambridge University Press, Cambridge 1996 (455 pp. ISBN ) W. Riegler/CERN 8

9 Creation of the Signal Charged particles traversing matter leave excited atoms, electron-ion pairs (gases) or electrons-hole pairs (solids) behind. Excitation: The photons emitted by the excited atoms in transparent materials can be detected with photon detectors like photomultipliers or semiconductor photon detectors. Ionization: By applying an electric field in the detector volume, the ionization electrons and ions are moving, which induces signals on metal electrodes. These signals are then read out by appropriate readout electronics. 4/2/2012 9

10 Detectors based on registration of excited Atoms Scintillators

11 Detectors based on Registration of excited Atoms Scintillators Emission of photons of by excited Atoms, typically UV to visible light. a) Observed in Noble Gases (even liquid!) b) Inorganic Crystals Substances with largest light yield. Used for precision measurement of energetic Photons. Used in Nuclear Medicine. c) Polycyclic Hydrocarbons (Naphtalen, Anthrazen, organic Scintillators) Most important category. Large scale industrial production, mechanically and chemically quite robust. Characteristic are one or two decay times of the light emission. Typical light yield of scintillators: Energy (visible photons) few of the total energy loss. e.g. 1 cm plastic scintillator, 1, de/dx=1.5 MeV, ~15 kev in photons; i.e. ~ photons produced. Only a fraction of which will be detected

12 Scintillation Detector Scintillation Energy deposition by ionizing particle production of scintillation light (luminescense) photodetector Scintillators are multi purpose detectors Calorimetry Time of flight measurement Tracking detector (fibers) Trigger counter Veto counter.. Requirements High efficiency Transparent (to its own radiation) Spectral range <-> Photodetectors Fast Two material types: Inorganic and organic scintillators Rise time (ns) Decay time (ns s) high light output lower light output but slow but fast

13 Detectors based on Registration of excited Atoms Scintillators Organic ( Plastic ) Scintillators Inorganic (Crystal) Scintillators Low Light Yield Fast: 1-3ns Large Light Yield Slow: few 100ns LHC bunchcrossing 25ns LEP bunchcrossing 25 s

14 Inorganic scintillators Inorganic crystalline scintillators (NaI, CsI, BaF 2...) Three effects: exitons (bound electron hole pairs), defects, activators (e.g. Tl) de/dx per scintillator photon for electrons: 25 (NaI) 300 (BGO) conduction band exciton band electron activation centres (im pu rities) scintillatio n ( nm ) luminescense quenching excitation traps E g hole valence band often 2 time constants: fast recombination (ns- s) from activation centre delayed recombination (phosphorescence, 100 s) Due to the high density and high Z inorganic scintillator are well suited for detection of charged particles, but also of. 2-3 orders of magnitude slower than organic scintillators (Exception: CsF (5 ns))

15 Inorganic Scintillators Light output of inorganic crystals shows some temperature dependence (From Harshaw catalog) PbWO 4 Practically no temperature dependence in organic scintillators (-60 to +20 degrees C) Liquid noble gases (LAr, LXe, LKr) A excitatio n ion izatio n A* A + collisio n w ith g.s. atoms excited molecule A 2 * A 2 + de-excitation and dissociation A A A 2 * UV 130nm (Ar) 150nm (Kr) 175nm (Xe) ion ized molecule recom bination e - also here one finds 2 time constants: few ns and ns, but same wavelength.

16 Inorganic Scintillators Properties of some inorganic scintillators -> Take a look at the detector section of the PDG book Photons/Me V PbWO , LAr ) / LKr ) / LXe ) / ) at 170 nm

17 PbWO 4 ingot and final polished CMS ECAL scintillator crystal from Bogoroditsk Techno-Chemical Plant (Russia).

18 Organic scintillators 2. Organic scintillators: Monocrystals or liquids or plastic solutions Scintillation is based on the 2 electrons of the C-C bonds. Emitted light is in the UV range. Monocrystals: naphtalene, anthracene, p-terphenyl. Liquid and plastic scintillators They consist normally of a solvent + secondary (and tertiary) fluors as wavelength shifters. Fast energy transfer via non-radiative dipole-dipole interactions (Förster transfer). shift emission to longer wavelengths longer absorption length and efficient read-out device

19 Absorption and Emission Stokes Shift If emission and absorption occur at the same wavelengths, most emitted photons would be absorbed within a short distance resulting in poor light output. Since excitation goes to higher vibrational states in the S 1 band, whereas decay goes from the base S 1 state, the emission spectrum is shifted to lower energies (longer wavelengths).

20 Organic scintillators (backup) Schematic representation of wave length shifting principle (C. Zorn, Instrumentation In High Energy Physics, World Scientific,1992) Some widely used solvents and solutes Liquid scintillators Plastic scintillators solvent Benzene Toluene Xylene Polyvinylbenzene Po lyvinylto luene Polystyrene secondary fluor p-terphenyl DPO PBD p-terphenyl DPO PBD tertiary fluor POPOP BBO BPO POPOP TBP BBO DPS After mixing the components together plastic scintillators are produced by a complex polymerization method. Some inorganic scintillators are dissolved in PMMA and polymerized (plexiglas).

21 Properties of Organic Scintillators yield/ NaI 0.5 Organic scintillators have low Z (H,C). Low detection efficiency (practically only Compton effect). But high neutron detection efficiency via (n,p) reactions.

22 Loss of light Light Collection Through absorption by scintillator material If I and I o are the intensities and L is the attenuation length The attenuation length is typically around 1 m (hence this effect is usually less important) Through the scintillator boundaries Wrap scintillator in foil (diffuse reflection like a Teflon film) Optical grease to couple to photo detector

23 Light Guides Photons are being reflected towards the ends of the scintillator. A light guide brings the photons to the Photomultipliers where the photons are converted to an electrical signal. Efficiency depends on angle of total internal reflections and conservation of phase space (Liouville Theorem) Scintillator Light Guide Photon Detector By segmentation one can arrive at spatial resolution. Because of the excellent timing properties (<1ns) the arrival time, or time of flight, can be measured very accurately Trigger, Time of Flight.

24 Typical Geometries: UV light enters the WLS material Light is transformed into longer wavelength Total internal reflection inside the WLS material transport of the light to the photo detector

25 Wavelength Shifting Use a fiber embedded in the scintillator instead of unwieldy light guides The fiber collects scintillation light, shifts it to longer wavelength and pipes it to a photo detector Evades the Liouville Theorem because shifting to longer wavelength cools the light (reduced phase space) Shifting from 450 nm to 500 nm corresponds to an energy shift of 0.28 ev Increased packing factor /.

26 Optical Fibers n arcsin n 1 d 4 3.1% Minimize n cladding Optical Fibers typ. 25 m core polystyrene n=1.59 typically <1 mm light transport by total internal reflection cladding (PMMA) n=1.49 Ideal: n = 1 (air), but impossible due to surface imperfections n 1 n 2 Multi-clad fibers Improved aperture d 4 5.3% Long(er) absorption length for visible light (> 10 m) core polystyrene n=1.59 cladding (PMMA) n= m fluorinated outer cladding n= m

27 Scintillating Fiber Tracker Scintillating plastic fibers Capillary fibers filled with liquid scintillator Planar geometries (end cap) Circular geometries (barrel) a) axial b) circumferential c) helical Advantages: High geometrical flexibility (R.C. Ruchti, Annu. Rev. Nucl. Sci. 1996, 46,281) Fine granularity Low mass Fast response (ns)

28 CERN WA84: Active (Fiber) Target Fiber Tracking Readout of photons in a cost effective way is rather challenging.

29 Photo Detectors Purpose: Convert light into detectable electronics signal Principle: Use Photoelectric Effect to convert photons to photoelectrons Standard Requirement: High sensitivity, usually expressed as Quantum efficiency Q.E. = N p.e. / N photons Main types of photodetectors: Gas based devices (see RICH detectors) Vacuum based devices (Photomultiplier) Solid state detectors Threshold of some photosensitive material TMAE,CsI TEA UV visible GaAs... multialkali bialkali E (ev) (nm)

30 Photo Multiplier Tube Operation Principle Photo emission from photo cathode Focusing, acceleration Secondary emission from dynodes e - photon (Philips Photonic) Gain Dynode gain g= 3-50 Total gain M N g i i 1 10 dynodes with g = 4 M = 4 10 ~ 10 6

31 Photo Cathode 3-step process Photo ionization of molecule Electron propagation through cathode Escape of electron into the vacuum Quantum Efficiency Most photo-cathodes are semiconductors Band model: Semitransparent photocathode glass PC e - Opaque photocathode e - PC substrate The photon energy has to be sufficient to bridge the band gap E g, but also to overcome the electron affinity E A, so that the electron can be released into the vacuum.

32 Typical Quantum Efficiencies Q.E. Bialkali SbK 2 Cs SbRbCs Multialkali SbNa 2 KCs Solar blind CsTe (cut by quartz window) Typical efficiency for photon detection: < 20% For very good PMs: registration of single photons possible. (Philips Photonic) Transmission of various PM windows

33 Energy Resolution of PMTs The energy resolution is determined mainly by the fluctuation of the number of secondary electrons emitted from the dynodes. n e Poisson distribution: P( n, m) m! Relative fluctuation: Fluctuations are the largest when n is small -> first dynode n Typical dynode materials: BeO(Cs), Cs 3 Sb, MgO; negative electron materials such as GaP(Cs) higher emission yield but more difficult to fabricate n n m n n 1 n Single photons. Pulse height spectrum of a PMT with Cu-Be dynodes. Pulse height spectrum of a PMT with NEA dynodes. 1 p.e. counts 1 p.e. (Philips Photonic) counts 2 p.e. 3 p.e. (H. Houtermanns, NIM 112 (1973) 121)

34 More on resolution Typical NaI(Tl) system (from H. Spieler) 511 kev gamma ray photons in scintillator photons at photocathode 3000 photoelectrons at first dynode 3x10 9 electrons at anode 2 ma peak current Resolution of energy measurement determined by statistical variance of produced signal quanta. E 1 2% r. m. s 5% FWHM E 3000

35 Dynode Configurations Many different dynode configurations have been developed to reduce size, or improve gain, uniformity over large photocathode diameters, transit time and transit time spread. traditional New micro-machined structures (Philips Photonics) position sensitive PMT s PM s are in general very sensitive to B-fields, even to earth field (30-60 T). -metal shielding required.

36 Continuous Multiplier Structure Channel Electron Multiplier Microchannel Plates Microchannel Plate Lead glass plate Fast timing Low time dispersion Image Amplifier holes Gain factors

37 What to expect an example Some parameters for a typical plastic scintillation counter: energy loss in plastic scintillator: 2MeV/cm scintillation efficiency of plastic: 1 photon/100 ev collection efficiency (# photons reaching PMT): 0.1 quantum efficiency of PMT 0.25 What size electrical signal can we get from a plastic scintillator 1 cm thick? A charged particle passing perpendicular through this counter: deposits 2MeV which produces 2x10 4 s of which 2x10 3 s reach PMT which produce»500 photo-electrons Assume the PMT and related electronics have the following properties: PMT gain = photo-electrons produce 5x10 8 electrons or q = 8x10-11 C Assume charge is collected in 50nsec (5x10-8 s) current = dq/dt = (8x10-11 coulombs)/(5x10-8 s) = 1.6x10-3 A Assume this current goes through a 50 resistor V=IR=(50 )(1.6x10-3 A)=80mV (big enough to see with Oscilloscope) So a minimum ionizing particle produces an 80mV signal.

38 Efficiency of this counter What is the efficiency of the counter? How often do we get no signal (zero photo electrons (PE))? The prob. of getting n PE s when on average <n> are expected is a Poisson process: P( n) n n e n! n The prob. of getting 0 photons is e -<n> =e -500 ~0. So this counter is»100% efficient. Note: a counter that is 90% efficient has <n>=2.3 PE s

39 Time dependence of emitted light Non-radiative transfer of energy from vibrational states to fluorescent state Typical time: ns Decay of fluorescent state Typical time: 1 3 ns Rise with time constant r I t 1 e t / r Fall with time constant f I t e t / f Total pulse shape I t / t / t I e e o o f I gain QE N with N E / o r o absorbed Note: the rise time is usually increased substantially by subsequent components in the system and variations in path length in large scintillators E per

40 Voltage Divider Operational Aspects of PMTs Electron multiplication at the dynodes depends on the potential between successive dynodes. Potential distribution typically set by resistive divider Typically. PMTs are operates at ~ 2kV 8-14 stages -> V between dynodes Typically larger for first stages to improve collection PMTs have (almost) linear gain until saturation sets in. NEA dynodes (GaP(Cs)) do not exhibit saturation Linear response

41 Advanced PMTs Multi Anode PMT (Example: Hamamatsu R5900 series) Up to 8x8 channels. Size: 28x28 mm 2. Active area 18x18 mm 2 (41%). Bialkali PC: Q.E. = 20% at max = 400 nm. Gain Gain uniformity and cross-talk used to be problematic, but recently much improved. Flat Panel PMT (Hamamatsu) Excellent surface coverage (>90%) 8 x 8 channels (4 x 4 mm 2 / channel) Bialkali PC, Q 20%

42 Other Photon Detectors Photo Diodes Hybrid Photo Diodes Silicon Photomultiplier Visible Light Photo Counter Future Lecture Gas Photo Multiplier Cherenkov Detectors Liquid Noble Gases Cryogenic Detectors

43 Scintillation Counter Plateau Low voltage: very few counts With increasing voltage (gain) the number of counts rises sharply once the signal Pulses are above the discriminator threshold Regeneration effects (after pulsing etc) at higher voltages Scintillation counters are typically operated in the middle of the plateau

44 References used today Particle Detectors, CERN Summer Student Lecture 2008, W. Riegler Particle Detectors, CERN Summer Student Lecture 2003, C. Joram Radiation Detectors, H. Spieler Material from the books by Leo and Gruppen Particle Data Book